1. Field of Invention
The present invention relates to a light guide plate included in a backlight unit for illuminating a compact liquid-crystal display device mounted on a portable apparatus, such as a mobile phone or a personal digital assistant.
2. Description of the Related Art
Recently, a liquid-crystal display device is mounted on various electronic apparatuses, such as watches, phones, and personal computers. Of such electronic apparatuses, watches and mobile phones, which are used at night and in dark places, often include a backlight unit for illuminating the liquid-crystal display device. Backlight units are broadly divided into two types, direct type and edge light type. A direct-type backlight unit includes a liquid-crystal display plate, a light source, and a light diffusion plate interposed therebetween. In an edge-light-type backlight unit, a liquid-crystal display plate and a light guide plate are disposed opposite each other, and a light source is disposed adjacent to the light guide plate such that light enters the light guide plate from the periphery thereof. An exemplary edge-light-type backlight unit is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2002-42529.
As compared to the direct-type backlight unit, the edge-light-type backlight unit is advantageous in that it can have a reduced thickness and can reduce the effects of heat from the light source on the liquid-crystal display panel.
FIG. 6A, FIG. 6B, and FIG. 7 illustrate an edge-light-type backlight unit. FIG. 6A is a plan view of the edge-light-type backlight unit and FIG. 6B is a cross-sectional view taken along line VIB-VIB of FIG. 6A. FIG. 7 illustrates an optical path of light emitted from a light source of the backlight unit.
As illustrated, a light guide plate 2 and four light-emitting diodes (hereinafter referred to as “LEDs”) 3 are disposed inside a housing 1 of the backlight unit. The light guide plate 2 is a clear rectangular plate made of plastic. The LEDs 3 are disposed adjacent to a side surface 2a serving as an incident surface of the light guide plate 2. Light emitted from the LEDs 3 enters the light guide plate 2 from the side surface 2a and propagates inside the light guide plate 2 while being repeatedly reflected on a bottom surface 2b and a top surface 2c of the light guide plate 2. When, as illustrated, the bottom surface 2b of the light guide plate 2 is provided with many reflecting prisms having slant surfaces with angles P, the incident angle of light from the LEDs 3 with respect to the top surface 2c serving as a light-emitting surface gradually decreases as the light propagates while repeatedly being reflected on the top surface 2c and the bottom surface 2b. When the incident angle becomes smaller than a predetermined critical angle, the light is emitted from the light-emitting surface to the outside. While part of illuminating light is emitted through the bottom surface 2b to the outside, the light is reflected by a reflecting sheet 4, such as a silver glossy sheet or a white sheet, back to the light guide plate 2. Light emitted from the top surface 2c of the light guide plate 2 is directed by a prism sheet 5 toward a liquid crystal panel (not shown).
However, as illustrated in FIG. 9, there are dark areas 7 on the top surface 2c of the light guide plate 2 in the known backlight unit. The dark areas 7 are adjacent to spaces between the LEDs 3. The amount of light emitted from the top surface 2c is extremely small in the dark areas 7.
One reason for this is that light emitted from each LED 3 has directivity. In other words, light from each LED 3 radiates out with an emitting direction centered, and the amount of light decreases as a radiation angle θ such as that illustrated in FIG. 6A increases.
Another reason for the occurrence of the dark areas 7 is that when the radiation angle θ exceeds a certain value, the effects of the slant surfaces of the reflecting prisms are virtually cancelled out. Specifically, as described above, the slant surfaces of the reflecting prisms reflect the light from the LED 3 to make the incident angle of the light to the top surface 2c smaller so that the light is eventually emitted through the top surface 2c to the outside when the incident angle of the light to the top surface 2c becomes smaller than a predetermined critical angle. However, since the angle of a slant surface of a reflecting prism with respect to the light decreases as the radiation angle θ increases, the above-described effects of the slant surfaces of the reflecting prisms are virtually cancelled out when the radiation angle θ exceeds a certain value. FIG. 8 illustrates a ratio of the angle of the slant surface with respect to light incident on the slant surface at a radiation angle θ relative to the angle of the slant surface with respect to light incident on the slant surface in the emitting direction of an LED, i.e., at the angle θ of zero.